The present invention relates to an electron gun for a cathode-ray tube.
FIG. 1 shows the electrode configuration of a quadripotential-focus (QPF) electron gun of the cathode-drive type, which is one example of the prior art. FIG. 2 illustrates the principle of this QPF electron gun.
Referring to FIG. 1, the QPF electron gun comprises a stem 1, three cathodes 2, and a first grid 3, second grid 4, third grid 5, fourth grid 6, fifth grid 7, and sixth grid 8. The first and second grids 3 and 4 are control electrodes comprising flat plates with apertures for passage of the three electron beams emitted from the cathodes 2. The fourth grid 6 comprises a generally similar electrode. The third, fifth, and sixth grids 5, 7, and 8 comprise cylindrical electrodes with interior or end baffles having apertures for passage of the electron beams.
A fixed anode voltage E.sub.B is applied to the sixth grid 8. A focus voltage E.sub.F generally equal to 20% to 30% of the anode voltage is applied to the third and fifth grids 5 and 7. The fifth and sixth grids 7 and 8 form a main lens 12, as indicated in FIG. 2. A lower fixed voltage E.sub.G2 is applied to the second grid 4. The same voltage E.sub.G2 or another voltage lower than the focus voltage is applied to the fourth grid 6, forming the prefocus lens 11 indicated in FIG. 2. A still lower fixed voltage E.sub.G1 is applied to the First grid 3. Red, green, and blue video signal voltages, generally intermediate between E.sub.G1 and E.sub.G2 , are applied to the cathodes 2. The part comprising the cathodes 2 and the first, second, and third grids 3, 4, and 5 is referred to as the triode 14.
The operation of the QPF electron gun is illustrated schematically in FIG. 2. An electron beam 101 emitted from one of the cathodes 2 is brought to a crossover 13 by the first and second grids 3 and 4, then prefocused by the unipotential lens effect of the prefocus lens 11. The prefocused electron beam is then focused onto a screen 10 by the bi-potential effect of the main lens 12, forming a beam spot.
FIG. 3 shows the potential distribution in the triode 14 of this QPF electron gun, omitting the third grid 5. It is convenient to discuss the electron optics in FIG. 3 in terms of an equivalent optical lens system, shown in FIG. 4, comprising a convex lens 102, a concave lens 103, and a convex lens 104. The concave lens 103 has a divergent effect that aligns the virtual object points of rays 15 emitted from the periphery of the cathode 2 and rays 16 emitted from near the center of the cathode 2 to substantially the same virtual object point 17. (The virtual object point is the position of the object point as seen from the main lens 12; this position affects the focal length of the main lens 12.) Another effect of the concave lens 103 is to suppress movement of the virtual object point when the beam current is increased. The beam aberration of the triode 14 can thereby be reduced, a beam spot with a small diameter can be formed on the screen 10, and resolution can be improved.
FIG. 5 shows electron trajectories in the conventional QPF electron gun at a low beam current level, while FIG. 6 shows electron trajectories at a high current level. At the low current level, under ideal conditions with no aberration, a beam emitted from a single point would be Focused to a single point. Under actual conditions, at low current levels, spherical aberration in the main lens 12 causes peripheral rays to be brought to a shorter focus, resulting in a slight enlargement of the beam-spot diameter on the screen 10, as shown in FIG. 5. As the beam current increases, so does the spherical aberration of the main lens 12, leading to a great enlargement of the spot size on the screen 10 as shown in FIG. 6. The result of this so-called blooming effect is poor resolution on the screen.
One conventional solution to this problem has been to increase the thickness of the second grid 4, in order to reduce the aberration of the main lens 12. Other conventional solutions have been to reduce the diameters of the beam apertures in the third, fourth, and fifth grids 5, 6, and 7 or to increase the thickness of the fourth grid 6, in order to strengthen the prefocus lens 11, thereby reducing the beam diameter at the position of its center of deflection. These solutions do not lead to fundamental improvements, however, because they enlarge the diameter of the virtual object point 17, resulting in increased magnification, so that the beam-spot diameter is increased in the center of the screen 10.